The word "帐单" is formed by combining the character "帐", meaning account, which is itself composed of the radical "巾" (cloth, historically used for keeping records) and the phonetic component "长" for pronunciation, with the character "单", meaning list or sheet, originating from a pictograph denoting a simple document. Together, they create a compound word where "帐" refers to the financial record and "单" to the document, resulting in the meaning of a bill or invoice.